Remove warnings from `lib` code

Right now, anyone who uses the [Fuubar] Rspec formatter gets a list of warnings coming from ruby-progressbar. They look like this:

```
/Users/grayson/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/ruby-progressbar-1.6.0/lib/ruby-progressbar.rb:10: warning: `*' interpreted as argument prefix
/Users/grayson/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/ruby-progressbar-1.6.0/lib/ruby-progressbar/length_calculator.rb:42: warning: assigned but unused variable - rows
/Users/grayson/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/ruby-progressbar-1.6.0/lib/ruby-progressbar/components/estimated_timer.rb:69: warning: `*' interpreted as argument prefix
/Users/grayson/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/ruby-progressbar-1.6.0/lib/ruby-progressbar/components/rate.rb:52: warning: `*' interpreted as argument prefix
/Users/grayson/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/ruby-progressbar-1.6.0/lib/ruby-progressbar/base.rb:147: warning: private attribute?
/Users/grayson/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/ruby-progressbar-1.6.0/lib/ruby-progressbar/base.rb:147: warning: private attribute?
/Users/grayson/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/ruby-progressbar-1.6.0/lib/ruby-progressbar/base.rb:154: warning: method redefined; discarding old last_update_length
/Users/grayson/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/ruby-progressbar-1.6.0/lib/ruby-progressbar/formatter.rb:33: warning: instance variable @format_string not initialized
/Users/grayson/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/ruby-progressbar-1.6.0/lib/ruby-progressbar/components/timer.rb:32: warning: instance variable @stopped_at not initialized
/Users/grayson/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/ruby-progressbar-1.6.0/lib/ruby-progressbar/components/timer.rb:32: warning: instance variable @stopped_at not initialized
/Users/grayson/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/ruby-progressbar-1.6.0/lib/ruby-progressbar/formatter.rb:33: warning: instance variable @format_string not initialized
/Users/grayson/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/ruby-progressbar-1.6.0/lib/ruby-progressbar/components/timer.rb:32: warning: instance variable @stopped_at not initialized
/Users/grayson/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/ruby-progressbar-1.6.0/lib/ruby-progressbar/components/timer.rb:32: warning: instance variable @stopped_at not initialized
/Users/grayson/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/ruby-progressbar-1.6.0/lib/ruby-progressbar/length_calculator.rb:12: warning: instance variable @current_length not initialized
/Users/grayson/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/ruby-progressbar-1.6.0/lib/ruby-progressbar/components/timer.rb:32: warning: instance variable @stopped_at not initialized
/Users/grayson/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/ruby-progressbar-1.6.0/lib/ruby-progressbar/components/timer.rb:28: warning: instance variable @started_at not initialized
/Users/grayson/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/ruby-progressbar-1.6.0/lib/ruby-progressbar/components/timer.rb:32: warning: instance variable @stopped_at not initialized
```

This pull request fixes those warnings. You might want a minor version bump with it.

Thanks for all your hard work with this library! It's a pleasure to use. (=